On Woltjer's force‐free minimizers and Moffatt's magnetic relaxation

Abstract

In this note, we exhibit a situation where a stationary state of Moffatt's ideal magnetic relaxation problem is different than the corresponding force-free L 2 $L^2$ energy minimizer of Woltjer's variational principle. Such examples have been envisioned in Moffatt's seminal work on the subject and involve divergence free vector fields supported on collections of essentially linked magnetic tubes. Justification of Moffatt's examples requires the strong convergence of a minimizing sequence. What is proven in the current note is that there is a gap between the global minimum (Woltjer's minimizer) and the minimum over the weak L 2 $L^2$ closure of the class of vector fields obtained from a topologically nontrivial field by energy-decreasing diffeomorphisms. In the context of Woltjer-Taylor relaxation, our result shows that the Taylor state cannot be reached during the viscous relaxation in the perfectly conducting magneto-fluid, if the initial field has a nontrivial topology. The result also applies to any other relaxation process which evolves a divergence free field by means of energy-decreasing diffeomorphisms, such processes were proposed by Vallis et al. and more recently by Nishiyama.